core: Have WritePlan be explicit that it's non-functional right now
core: Have WritePlan be explicit that it's non-functional right now

This is temporarily broken until we implement the new plan file format,
since terraform.Plan is no longer serializable with gob. Rather than have
an error that seems like it needs immediate fixing, we'll be explicit
about it in the error message and focus our efforts on other test failures
for now, and return to implement the new file format later.

brandontosch/terraform,iljaskevic/terraform,iljaskevic/terraform,pryorda/terraform,VladRassokhin/terraform,tommynsong/terraform,heimweh/terraform,pryorda/terraform,heimweh/terraform,rnaveiras/terraform,iljaskevic/terraform,VladRassokhin/terraform,mattray/terraform,pdecat/terraform,hashicorp/terraform,ewbankkit/terraform,tommynsong/terraform,drebes/terraform,heimweh/terraform,gechr/terraform,tpounds/terraform,rnaveiras/terraform,youhong316/terraform,mattray/terraform,ewbankkit/terraform,pdecat/terraform,gechr/terraform,ewbankkit/terraform,drebes/terraform,apparentlymart/terraform,pdecat/terraform,luis-silva/terraform,apparentlymart/terraform,luis-silva/terraform,drebes/terraform,tpounds/terraform,pryorda/terraform,brandontosch/terraform,apparentlymart/terraform,nlamirault/terraform,tommynsong/terraform,tpounds/terraform,youhong316/terraform,heimweh/terraform,VladRassokhin/terraform,ewbankkit/terraform,brandontosch/terraform,drebes/terraform,nlamirault/terraform,nlamirault/terraform,pradeepbhadani/terraform,tpounds/terraform,luis-silva/terraform,pradeepbhadani/terraform,youhong316/terraform,apparentlymart/terraform,youhong316/terraform,mattray/terraform,luis-silva/terraform,pradeepbhadani/terraform,VladRassokhin/terraform,mattray/terraform,iljaskevic/terraform,brandontosch/terraform,apparentlymart/terraform,rnaveiras/terraform,pdecat/terraform,brandontosch/terraform,hashicorp/terraform,gechr/terraform,hashicorp/terraform,heimweh/terraform